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Ex 1. (3 pt) Prove by induction on dimension the following facts about the homology of a finite-dimensional CW complex X, using the observation that X

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USTC, School of Mathematical Sciences Winter semester 2018/19 Algebraic topology by Prof. Mao Sheng Exercise sheet 8

MA04311 Tutor: Lihao Huang, Han Wu 11 points

Posted online by Dr. Muxi Li

Ex 1. (3 pt) Prove by induction on dimension the following facts about the homology of a finite-dimensional CW complex X, using the observation that X

n

/X

n−1

is the wedge sum of n-spheres:

(a) If X has dimension n then H

i

(X) = 0 for i > n and H

n

(X) is free.

(b) H

n

(X) is free with basis in bijective correspondence with the n-cells if there are no cells of dimension n − 1 or n + 1.

(c) If X has k n-cells, then H

n

(X) is generated by at most k elements.

Ex 2. (2 pt) Show that the second barycentric subdivision of a 4-complex is a simplicial complex. Namely, show that the first barycentric subdivision produces a 4-complex with the property that each simplex has all its ver- tices distinct, then show that for a 4-complex with this property, barycentric subdivision produces a simplicial complex.

Ex 3. (2 pt) Show that a simplicial complex structure on the torus needs at least 14 triangles, 21 edges and 7 vertices. Write down this simplicial structure explicitly. [hint: using Euler formula for torus to get the relation of the number of triangles, edges and vertices.]

Ex 4. (2 pt) Let f : (X, A) → (Y, B) be a map such that both f : X → Y and the restriction f : A → B are homotopy equivalences.

(a) Show that f

: H

n

(X, A) → H

n

(Y, B) are isomorphism for all n.

(b) For the case of the inclusion f : (D

n

, S

n−1

) → (D

n

, D

n

− {0}), show that f is not a homotopy equivalence of pairs—there is no g : (D

n

, D

n

− {0}) → (D

n

, S

n−1

) such that f g and gf are homotopic to the identity through maps of pairs.

Ex 5. (2 pts) Recall that for a good pair (X, A), the relative homology H

n

(X, A) can be expressed as reduced absolute homology ˜ H

n

(X/A). Now, for an arbitary pair (X, A), show that H

n

(X, A) ∼ = ˜ H

n

(X ∪

A

CA), where CA is the cone (A × I)/(A × {0}), and ∪

A

means identifying A ⊂ X with the base of the cone A × {1}.

Note: Please hand in this homework on 28

th

Nov. 2018.

1

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